There is a continuous increase in number of cases in which electric motors are driven by using inverters of pulse-width modulation method (hereinafter referred to as “PWM method”). In such a case where a motor is driven with an inverter of the PWM method, a difference in electric potential occurs between an outer ring and an inner ring of a bearing (referred to as “shaft voltage”) since an electric potential at a neutral point of a winding does not stay at 0 (zero) volt. The shaft voltage includes a high-frequency component attributable to switching operation, and micro electric current flows inside the bearing when the shaft voltage reaches a breakdown voltage of an oil film inside the bearing, thereby causing electrolytic corrosion in the bearing. When the electrolytic corrosion progresses, a phenomenon of wavy wear appears on the outer ring, inner ring or balls of the bearing, thereby leading to abnormal sound noise which becomes one of the main causes of troubles of the motor.
A number of techniques have hitherto been proposed to suppress the electrolytic corrosion for the purpose of avoiding such troubles. In an example of International Publication No. 2009/113311, electrolytic corrosion is suppressed by decreasing the shaft voltage with a dielectric layer provided in a rotor.
In conventional techniques like the above example, however, there is a limitation in setting the shaft voltage appropriately though it has an effect of reducing a high frequency voltage induced in an inner ring of the bearing by making use of an electrostatic capacitance of the dielectric layer.
In other words, the shaft voltage can be set appropriately if the electrostatic capacitance provided by the dielectric layer is freely changeable. Such a change in the electrostatic capacitance is possible by changing a dielectric constant of an insulation resin that forms the dielectric layer, changing a thickness of the insulation resin, or changing an area between both cores. However, sizes of motors used in electric apparatuses are normally determined according to individual types of the electric apparatuses, and sizes of their rotors are generally standardized such that it is difficult to substantially alter the sizes, or to change the shape of dielectric layers in a flexible manner. Although a resin material and the like need to be changed in order to change a dielectric constant of the insulation resin that forms the dielectric layer, it is not easy to make any change in the material since such a change makes it necessary to confirm other points such as the strength. The conventional technique thus has a limitation in setting the dielectric layer in a manner to make the shaft voltage become optimum.